Screen grid tube



l 1 H. o. PETERSON 1,873,026

seamen GRID TUBE Filed Dec. 20. 1928 i3 qt."

F2 Cad If I r l r I l I i i I l l l i l I I I I l I HAROLD PETERSON 35hf' 'qbtomm Patented Aug. 23, p 1932 UNITED STATES PATENT? OFFICE HAROLD0. PETERSON, OF RIVEBHEAD. NEW YORK, ASSIGNOR TO RADIO CORPORATION OFAMERICA, A CORPORATION OF DELAWARE SCREEN GRID TUBE Application filedDecember 20, 1928. Serial No. 327,205.

This inventionrelates to tubes and more particularly to tubes of thescreen grid type.

In radio work the high frequencies of the electrical oscillations haveresulted in efforts to neutralize the capacity couplingexistin'g betweenthe various electrodes of the electron discharge tube. As we increasethe frequency with which we work the greater is the effect of thiscapacity and the greater is the extent to which we must shield the electrodes in order toavoid undesirable coupling. One of the means forreducing the capacity coupling between the electrodes is to insert ascreen between the plate and grid to avoid the undesirable feedback ofenergy from the plate to the grid. At the extremely high frequenciesused, for example, in five meter work, it has been found that a screenas ordinarily applied is not suflicient, This is due to the fact thatthe leads to this screen, though in the form of pieces of straight wire,have appreciable self-inductance, making it difficult to apply a radiofrequency short circuit between screen and filament. v

To avoid this inductance, I have introduced a very short lead to thescreen grid which has a terminal on top of the tube. The shield grid isgrounded for radio frequencies through this short lead, and it has beenfound that this more effective grounding reduces the inter-electrodecoupling between plate and control grid.

The invention is more fully described in connection with theaccompanying figure which shows a diagrammatic arrangement of thevarious elements in a tube according to my invention.

In the figure is shown a tube with a bulb or envelope of the usual typeenclosing a filament 2 having lead wires 3 and 4 sealed into the pressof a reentrant stem of the usual type and extending through the stemtube to the base of the tube where they are connected to terminals 5 and6. About the filament is shown the input electrode or grid 7 This gridhas a lead 8 connecting with the terminal 9 on the top of the tube. Theplate 12 surrounds the grid and has a lead wire 13 which is sealed intothe press of the stem and connects with the terminal 14 also on the baseof the tube. The shield grid 15' acts as a screen between the plate 12and the input electrode or grid 7. The shield 15 has a long lead 16sealed into the press of the stem and extending through the stem tube tothe base of the tube to a bottom terminal 17.

As is well known, the shield or screen re- "duces the effective capacitybetween the plate electrode 12 and the input or grid electrode 7 I Thevariations in the plate voltage on the electrode 12 would be transferredby capacity coupling to the grid 7 if there were no shield 15 betweenthem to effect a grounding of the capacitive current. In tubes of thisconstruction as commonly used the screen or shield 15' is kept at aconstant potential, usually a positive potential, through the lead 16.

p The outer portion 15 and the inner portion 15 ofthe screen areelectrically connected to each other through the metal disc 20.

It has been found that at these extremely high frequencies, such as areused for wave lengths of five meters or less, the reactance of the longlead 16 was such that the tube persistently oscillated when the plateand grid circuits were brought into resonance. The construction of thetube Wlll not permit the shortening of the lead 16, which extends fromthe lower edge of the screen 15 through the press and stem tube to thebase terminal 17, and which ordinarily has a length of between two andthree inches. To reduce the reactance of this screen grid lead I haveprovided a short additional lead 18 going out through the wall of theglass envelope at a point remote from the stem and preferably near thetop to a top terminal 19. The length of this lead 18 is approximately aninch. With this construction the positive biasing potential is appliedthrough the usual lead 16, and the ground connection to the screen 15 isapplied through the shortlead 18. The more effective grounding hasresulted in a more effective screening action of the shield 15 and hasreduced oscillation of the tube. This permits working the tube at higherfrequencies. As in the usual type of screen grid tube both the positivebiasing potential and the ground are applied through the lead 16, so inthis construction according to my invention both the positive biasingpotential and the ground may be applied through the short lead 18 andthe use of the long lead 16 may be dispensed with entirely. In suchaconstruction the long lead 16 may be omitted or else no connection maybe provided for the terminal 17, both the grounding andthe positivepotential being applied to the short lead 18 through the terminal 19.

This invention may be applied to tubes used for either transmitting orrcceivin". The particular arrangement of electrodes shown in the figureis merely illustrativeand the electrodes may have forms and relativepositions other than those shown.

Claims:

1. A vacuum tube comprising an electron emitting cathode, a plate, agrid between the cathode and plate, and a screen between the plate andthe grid, the screen being provided ith two terminals, one at the bottomof the tube and one at the top of the tube, a long lead connecting saidscree to said terminal at the bottom of the tube and a short lead notsnbs'tantiall; longer than one half the length of said long leadconnecting said screen to said terminal at the top of said tube.

2. A vacuum tube consisting of an electron emitting cathode, a grid, aplate, and ascreen between the plat and grid, the cathode and platebeing connected to terminals at the bottom only of the tube, and thescreen and grid being connected to terminals near the top of the tube.

3. A, electron discharge device comprising a bulb having a reentrantstem Witlra press on the inner end, a catl'iode, grid, and plate mountedon said stein, a screen mounted on said stem to be bet-ween said gridand said plate, and a lead shorter than the distance from the end ofsaid screen adjacent said press to the outer end of said stem extendingthrough the wall of said bulb from the other end of said screen.

4. An electron discharge device comprising-a bulb having a reentrantstem with a press on the inner end, a cathode and a plate mounted onsaid stem, leads for said cathode and plate extending thru said press tothe outerend of said stem, :1 grid mounted adJacent said cathode, ascreen mounted between said grid and said plate, and a lead shorter thaneither said cathode lead or plate lead extending thru the wall of saidbulb remote from said stem and connected to said screen.

HAROLD O. PETERSON.

